In some applications, there is a desire to add relatively small amounts of additive fluid in precise proportion to relatively large amounts of a host fluid. By way of non-limiting example, there are applications where it is desired to add relatively small amounts of a marker dye (an additive fluid) in precise proportion to relatively large amounts of fuel (a host fluid). For example, in some applications there is a desired to add marker dye to fuel in a ratio of 14 ppm±1 ppm or 0.0014%±0.0001%. In some such applications, it may be desirable that the additive fluid and the host fluid be separately stored and that the additive fluid be added to the host fluid as and when the combined fluid is required (e.g. while the host fluid is being dispensed for a particular application). In the case of marker dye and fuel, for example, separate storage and addition of the marker dye to the fuel at the time of dispensing can eliminate the need to store and/or transport dyed fuel separately from undyed fuel. In some applications, it can be desirable to add the additive fluid to the host fluid at a controllable volumetric flow rate. By way of non-limiting example, when a host fluid is being dispensed at a particular flow rate, it can be desirable to control the flow rate at which additive fluid is added to the host fluid, to maintain or track a desired relationship between the dispensing flow rate of the host fluid and additive flow rate of the additive fluid. In some applications, it can be desirable to add the additive fluid to the host fluid at a controllable volumetric flow rate that is controllable in relatively small increments and is controllable over a relatively wide range of flow rates. By way of non-limiting example, when fuel can be dispensed at a rate in a range of x-y milliliters per second (ml/s) and it is desired to add dye at a flow rate of z % of the fuel dispensing rate at an accuracy of ±w %, then it is desirable that the controllable range of dye delivery be at least 10−2 x·z ml/sec-10−2 y·z ml/sec and that this dye delivery be controllable increments as small as 10−4 x·z·w ml/sec.